Minute volume and carbon dioxide clearance as surrogates for EtCO2 in automatic ventilation

ABSTRACT

A method for automatically controlling ventilation of a patient includes receiving a target expiratory CO 2  concentration, measuring an actual expiratory CO 2 , and comparing the actual expiratory CO 2  concentration to the target expiratory CO 2 . A ventilation rate for the patient is then calculated based on the comparison of the actual expiratory CO 2  concentration and the target expiratory CO 2  in order to maintain the actual expiratory CO 2  within a predetermined range of the target expiratory CO 2 . The patient is then automatically ventilated based on the calculated ventilation rate.

BACKGROUND

Current ventilation systems require control by a clinician, who inputs control values for the ventilator, including ventilation rate and gas values. For example, during surgery and in an intensive care unit a patient is ventilated mechanically by a mechanical ventilator that is controlled by a clinician. In such applications, end tidal CO₂ (EtCO₂) is often measured to evaluate ventilation adequacy and to supervise patient status. Changes in EtCO₂ can be an indication of metabolic and/or hemodynamic changes in a patient, and thus EtCO₂ is a valuable monitoring parameter to clinicians.

Currently available systems that require clinician control are prone to user error. Thus, it is desirable to create an automated ventilation system that eliminates the requirement of clinician control. If administered properly, automatic ventilation control can eliminate user error and provide a safer ventilation control to a patient. However, choosing the right control variables and effectuating the automatic control algorithms is challenging because the human respiratory system is a complicated system with many variables that must be accounted for.

SUMMARY

In one embodiment, a method for automatically controlling ventilation of a patient includes receiving a target expiratory CO₂ concentration, measuring an actual expiratory CO₂, and comparing the actual expiratory CO₂ concentration to the target expiratory CO₂. A ventilation rate for the patient is then calculated based on the comparison of the actual expiratory CO₂ concentration and the target expiratory CO₂ in order to maintain the actual expiratory CO₂ within a predetermined range of the target expiratory CO₂. The patient is then automatically ventilated based on the calculated ventilation rate.

Another embodiment of a method for automatically controlling ventilation of a patient includes receiving a target EtCO₂ for the patient, receiving an alveolar minute volume value for the patient, measuring an expiratory CO₂ in a gas expired from the patient, and then calculating an actual EtCO₂. The actual EtCO₂ is compared to the target EtCO₂. If the actual EtCO₂ is not within a predetermined range of the target EtCO₂, then the alveolar minute volume value is adjusted and the patient is automatically ventilated using the adjusted alveolar minute volume value. A change in patient status is then indicated.

An embodiment for a system for automatically ventilating a patient includes a ventilator, a gas analyzer, a controller, and a display. The controller automatically controls the ventilator to ventilate the patient. The controller receives a target EtCO₂ for the patient and an initial alveolar minute volume value for the patient. The controller also receives the actual EtCO₂ from a gas analyzer. The controller then compares the actual EtCO₂ to the target EtCO₂. If the actual EtCO₂ is not within a predetermined range of the target EtCO₂, then the controller calculates an adjusted alveolar minute volume value and automatically ventilating the patient based on the adjusted alveolar minute volume value.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the best mode presently contemplated of carrying out the disclosure. In the drawings:

FIG. 1 depicts one embodiment of an automatic ventilation system.

FIG. 2 depicts a graph comparing measured EtCO₂ changes where MV_(alv) is held constant to measured changes in the MV_(alv) value when EtCO₂ is automatically maintained at a constant level according to the methods and systems disclosed herein.

FIG. 3 depicts one embodiment of a method of automatically ventilating a patient.

FIG. 4 depicts another embodiment of a method of automatically ventilating a patient.

FIG. 5 depicts an exemplary output display presenting an MV_(alv) value and a target EtCO₂ value for a patient over time.

FIG. 6 depicts an embodiment of a method of automatically ventilating a patient.

FIG. 7 depicts yet another embodiment of a method of automatically ventilating a patient.

FIG. 8 depicts an exemplary output display presenting a VCO₂ value and a target EtCO₂ value for a patient over time.

DETAILED DESCRIPTION OF THE DRAWINGS

The present inventors have recognized that in automatic ventilation the expiratory CO₂ concentration, such as EtCO₂, can be used as a control parameter. This means that the controller may adjust ventilation automatically to maintain EtCO₂ at a given target value. However, the inventors have recognized that automatically maintaining EtCO₂ at a given target value via an automated ventilation system could be problematic because the system controller may not be capable of determining the cause of changes in EtCO₂, as it cannot differentiate between all of the potential reasons that may lead to EtCO₂ changes. Furthermore, since the controller would keep EtCO₂ at a constant value by adjusting other ventilation parameters, a clinician would not see changes in a patient's EtCO₂ value, and thus valuable information about a patient's physiological status may be lost. Adequate information must be provided to a clinician indicating a patient's metabolic and hemodynamic states. The present inventors have realized that ventilation rate can be used alongside target expiratory CO₂ concentration to indicate changes in a patient hemodynamic and metabolic status. In the present invention, expiratory CO₂ concentration is kept constant with automatic control. In a preferred embodiment, EtCO₂ is used as the parameter indicating expiratory CO₂ concentration.

Automatically ventilating a patient by using EtCO₂ as a control parameter differs from currently available or practiced ventilation techniques and systems. Traditionally, clinicians control ventilation by setting a ventilation rate on the ventilator which is kept constant throughout the ventilation process unless the clinician manually adjusts the ventilation rate. For example, minute volume (MV), including minute alveolar volume (MV_(alv)), is kept constant at a value set by a clinician. The clinician may change the input value, for example, upon seeing a need to compensate for a change in a patient's EtCO₂ value, which is allowed to fluctuate and is monitored as an output value indicating patient status. For example, in currently available ventilation control methods EtCO₂ may vary as much as +/−10% from an optimum or anticipated value before a clinician manually adjusts ventilation parameters to compensate for an EtCO₂ change. Thus, in current ventilation systems, ventilation rate is set by a user and changes in expiratory CO₂ concentration, such as the EtCO₂ value, indicate a change in the hemodynamic and/or metabolic status of the patient.

The present inventors recognized that the challenges posed by deficiency of information when expiratory CO₂ concentration is kept constant can be overcome by reporting changes in ventilation rate, such as changes in MV_(alv), to indicate a change in patient hemodynamic and/or metabolic status. By the present invention, patient hemodynamic and metabolic monitoring during ventilation where ventilation rate is automatically adjusted to minimize variation in measured patient expiratory CO₂ concentration is conducted by adjusting the ventilation rate to maintain an approximately constant expiratory CO₂ concentration and reporting the change in the ventilation rate to indicate a change in the patient hemodynamic and/or metabolic status. For example, in automatic ventilation control according to the present invention, a target EtCO₂ value may be set by a clinician and changes in MV_(alv) may be reported to indicate a change in patient hemodynamic and/or metabolic status. In one embodiment of such a system, the target EtCO₂ value automatically maintained by a ventilation control system is shown together in the same view as the calculated MV_(alv) value so that a clinician can monitor a patient's status. Because MV_(alv) has always been held constant in current and prior ventilation systems, it is not obvious that MV_(alv) can be presented as a patient parameter.

The present inventors also recognized that the challenges posed by deficiency of information when expiratory CO₂ concentration is kept constant can be overcome by reporting changes in carbon dioxide clearance (VCO₂), to indicate a change in patient hemodynamic and/or metabolic status. For example, in automatic ventilation control according to the present invention, a target EtCO₂ value may be set by a clinician and changes in VCO₂ may be reported to indicate a change in patient hemodynamic and/or metabolic status. In one embodiment of such a system, the target EtCO₂ value automatically maintained by a ventilation control system is shown together in the same view as the VCO₂ value so that a clinician can monitor a patient's status.

For the purposes herein, it should be appreciated that carbon dioxide clearance may be referred to as carbon dioxide elimination or carbon dioxide production, and the abbreviation VCO₂ should refer to all of the aforementioned terms. It should further be appreciated that carbon dioxide clearance may be measured through integration of the product of breathing gas flow and its CO₂ concentration, or alternatively, approximated using the measured or target EtCO₂ multiplied with the alveolar ventilation (MV_(alv)) as VCO₂=EtCO₂×MV_(alv).

As shown in FIG. 1 , an automatic ventilation system 1 comprises a ventilator 3 connected to a patient 9 through a breathing circuit 6. The breathing circuit may include an endotracheal tube or a mask to connect the breathing circuit 6 to the patient 9. A gas analyzer 7 is connected to the breathing circuit 6 such that it can analyze gas expired by the patient 9. The gas analyzer 7 may be, for example, a time capnogram. Time-based capnography, such as EtCO₂ monitoring, is widely used in ventilation applications, including anesthesia and ICU applications. Time capnographs make use of main-stream sensors or side stream sensors. Volumetric capnography, which provides direct measurement of VCO₂, requires specialized equipment that is not widely available. Accordingly, one benefit of the presently disclosed method and system is that patient CO₂ monitoring is provided using the widely available time-based capnography equipment, as MV_(alv) estimation does not require any extra hardware than that available on currently available ventilation systems. Volumetric capnography, on the other hand, requires a separate device that is often not available in many ventilation applications, such as in the operating room or in the ICU. Accordingly, the gas analyzer 7 indirectly reflects the production of CO₂ by the patient's tissues and the patient's circulatory transport of CO₂ to the lungs.

The ventilator 3 is controlled by controller 5, which may be any type of controller capable of automatically controlling the ventilator 3. For example, the controller may be control software integrated into the control module in an anesthesia machine. Alternatively, the controller may be control software stored and executed on a separate computing device, such as a laptop, used in conjunction with an anesthesia machine or other ventilator. It is to be recognized that the controller 5 may be any combination of software and hardware implemented to perform the methods disclosed here in, and may include one or more processors that are communicatively connected so as to cooperate in providing a control function. The controller 5 may further comprise a microprocessor and other circuitry that retrieves and executes software from a storage system. Examples of processors include general purpose central processing units, application specific processors, and logic devices, as well as any other type of processing device, combinations of processing devices, or variations thereof. The storage system can comprise any storage media readable by processing system, and capable of storing software for the execution of control algorithms. The storage system can include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, program modules, or other data.

A user interface 11 is provided to allow an operator 17 to interact with and/or monitor the controller 5 and the ventilator 3. The user interface is connected to the ventilator 3 and/or the controller 5 via connection 12. Connection 12 may be any wired or wireless connection between the ventilator 3 and the user interface 11. For example, the connection 12 may utilize Wi-Fi, Bluetooth, or wireless medical telemetry service (WMTS) technology, or any other wireless technology available. In other embodiments, the connection 12 may be a physical wired data connection. The user interface 11 may have a display 13 and an input device 15. The user interface 11, display 13, and input device 15 may be any device or devices that allow an operator 17 to interface with the ventilator 1 and controller 5 to oversee and control the automatic ventilation of a patient. The user interface 11 may be integrated into an anesthesia cart or provided separate from an anesthesia cart. For example, the user interface 11 may be integrated into an anesthesia cart as a touch screen that acts as a display 13 to display monitoring data from the patient 9 and output data from the automatic ventilation system 1, and also to allow an operator 17 to input control commands to the system. In other embodiments, the user interface 11 may include a mouse, a keyboard, a voice input device, any touch input device for receiving a gesture from a user, a motion input device for detecting non-touch gestures and other motions by a user, and other comparable input devices and associated processing elements capable of receiving user input from a user. Output devices such as a video display or graphical display can display an interface further associated with embodiments of the system and method as disclosed herein. Speakers, printers, haptic devices and other types of output devices may also be included in the user interface 11.

Expired CO₂ concentration can be related to the expired volume over time to yield CO₂ elimination (VCO₂) rate, which is an important measure of patient metabolism and hemodynamic status. For example, sudden changes in CO₂ elimination during lung or heart surgery often imply important changes in cardiorespiratory function. For example, changes in a patient's EtCO₂ value or MV_(alv) value would be used by clinicians to provide indication of a change in cardiorespiratory function. Similarly, when one of these parameters (EtCO₂ or MV_(alv)) is maintained constant, changes in the other parameter may be used to detect lung conditions such as a pulmonary embolism, hemodynamic conditions such as a drop in cardiac output, or metabolic conditions such as metabolic hyperactivity. The automatic ventilation system 1 includes controller 5 configured to keep the patient's expiratory CO₂ concentration at a target value by controlling the ventilation rate. In the present disclosure, the CO₂ concentration measurement is exemplified as EtCO₂ and ventilation rate is exemplified as alveolar minute volume (MV_(alv)). However, it is contemplated that other parameters may be used to gauge patient ventilation rate, including minute volume. It is further contemplated that other parameters indicating expiratory CO₂ concentrations may be used other than EtCO₂.

Both MV_(alv) and EtCO₂ measurements track carbon dioxide production (VCO₂). The MV_(alv) value is an effective patient monitoring value because it reflects changes in patient status very well. MV_(alv) is the net effect of all of the ventilation rate settings and variables, including tidal volume, respiration rate, and dead space. Thus, MV_(alv) is a sensitive variable. Further, the inventors have recognized that MV_(alv) has more fluctuation and sensitivity than EtCO₂ for the same patient situation, and thus may provide more immediate detection of changes in patient metabolic or hemodynamic status. MV_(alv) has high sensitivity because it is primarily a controlled variable.

MV_(alv) may be defined as MV_(alv)=MV−DS×RR wherein MV is minute volume, DS is estimated dead space for the patient, and RR is respiration rate. VCO₂ can be depicted as a product of both MV_(alv) and EtCO₂ where VCO₂=EtCO₂×MV_(alv). Thus, carbon dioxide production trends can be reported and visualized using MV_(alv) values in place of EtCO₂ values.

The technical feasibility of using MV_(alv) as a patient parameter and as a variable for controlling EtCO₂, and thus VCO₂, was tested and the results of the feasibility testing are exemplified in FIG. 2 . FIG. 2 depicts a graph 20 comparing measured EtCO₂ changes where MV_(alv) is held constant (line 24), as is done in current ventilation systems, and measurement of changes in the MV_(alv) value when EtCO₂ is automatically maintained at a constant level (line 22) according to the methods and systems disclosed herein. In FIG. 2 , the x axis 26 depicts time, while the y axis 27 and 28 depicts either the MV_(alv) value 27 corresponding to line 22 or the EtCO₂ value 28 corresponding to line 24. Tests were performed with a physical patient simulator which was ventilated with an anesthesia machine and automatic ventilation control. The patient simulator CO₂ production was changed to simulate a change in patient metabolism. FIG. 2 depicts the results, which show that a change in the MV_(alv) value effectuated by the automatic controller was reflected in the EtCO₂ value (line 22). More specifically, a 20% change in MV_(alv) value (line 22) corresponded to approximately a 20% change in the EtCO₂ value (line 24).

FIG. 3 depicts one embodiment of a method of automatic ventilation control 35. A target expiratory CO₂ concentration is received at step 36. For example, the target expiratory CO₂ concentration value may be set by an operator, such as a clinician. At step 38, CO₂ levels are measured in a gas expired by a patient to determine an actual expiratory CO₂ level. At step 40, the actual expiratory CO₂ concentration is compared to the target CO₂ concentration. Then, a ventilation rate may calculated at step 42 based on the comparison between the actual expiratory CO₂ concentration and the target expiratory CO₂ concentration. For example, if the actual expiratory CO₂ concentration equals the target expiratory CO₂ concentration, then the ventilation rate may be maintained at a current ventilation rate. Alternatively, the ventilation rate may be increased if the actual expiratory CO₂ concentration is higher than the target expiratory CO₂ concentration. At step 44, the system automatically ventilates the patient at the ventilation rate determined or calculated at step 42. Then, the ventilation rate calculated at step 42 is displayed on a display at step 46, along with the target expiratory CO₂ concentration.

In other embodiments, minute volume or tidal volume may be adjusted in volume controlled ventilation in response to a determination that the actual expiratory CO₂ concentration is not equal to the target expiratory CO₂ concentration. In a pressure controlled ventilation system, minute volume or inspired pressure may be adjusted instead of or in addition to ventilation rate.

In another embodiment, such as that depicted in FIG. 6 , method 135 a may include the steps 36, 38, 40, 42, and 44 as described with respect to FIG. 3 . Then, at step 146, the carbon dioxide clearance (VCO₂) is displayed along with the target expiratory CO₂ concentration on a display.

FIG. 4 demonstrates another embodiment of a method for automatically ventilating a patient 35. At step 51, a target EtCO₂ value is received. For example, the target EtCO₂ value may be set by an operator. At step 53, an MV_(alv) value is received, which is the alveolar minute volume currently delivered to the patient. An actual EtCO₂ is determined at step 57 for the patient, such as based on a measurement of CO₂ in gas expired by the patient. For example, the gas analyzer measures expiratory CO₂ in a gas expired from the patient and then calculates an actual EtCO₂ based on the expiratory CO₂ measurement. At step 59, the actual EtCO₂ is compared to the target EtCO₂ to determine whether the actual EtCO₂ is approximately equal to the target EtCO₂. For example, the system may determine whether the actual EtCO₂ is within a 0.01% predetermined range of the target EtCO₂. In another embodiment, the system may determine whether the actual EtCO₂ is within a 0.5% predetermined range of the target EtCO₂. Preferably, the system determines that the actual EtCO₂ is not approximately equal to the target EtCO₂ if the difference between the actual and target EtCO₂ values is greater than 0.5%. However, it is contemplated that the system may be more sensitive and require the actual EtCO₂ to be closer than 0.5% to the target EtCO₂ in order to be considered approximately equal. In another embodiment, the system may require the actual EtCO₂ to be precisely equal to the target EtCO₂ within the measurement sensitivity of the system. If the actual EtCO₂ is equal to or approximately equal to the target EtCO₂ at step 59, then no change is made to the patient ventilation at that time and the monitoring and control method restarts at step 53 where the current MV_(alv) value is received for the current patient ventilation.

If the actual EtCO₂ is not equal to or approximately equal to the target EtCO₂, then the system continues to step 61 where an adjusted MV_(alv) value is calculated. The adjusted MV_(alv) value may be calculated based on the difference between the actual EtCO₂ value and the target EtCO₂ value. The MV_(alv) value is adjusted with the goal of bringing the actual EtCO₂ value within the predetermined range of the target EtCO₂ value. Then, at step 63, the patient ventilation is automatically adjusted to comport with the adjusted MV_(alv) value calculated at step 61. For example, the MV_(alv) value is translated into ventilation parameters for the patient, such as respiration rate or respiration volume, and then the patient ventilation is changed to reflect the adjusted ventilation parameters. Once the ventilation parameters are automatically adjusted at step 63, the adjusted MV_(alv) values are displayed at step 67, such as by display 12, and the control method returns to step 53 where the adjusted MV_(alv) value is received as the new current MV_(alv) value for the patient respiration.

At step 67, the adjusted MV_(alv) value and target EtCO₂ may be displayed to an operator, such as a clinician. For example, the display, such as display 12, may provide the MV_(alv) value over time, and the display may be updated at step 67 to reflect the adjusted MV_(alv) value. FIG. 5 depicts one exemplary display presenting an MV_(alv) value and a target EtCO₂ value for a patient over time. The upper portion of the exemplary display presents an MV_(alv) trend line 76 showing the MV_(alv) value over a time scale 74. The MV_(alv) magnitude scale 72 is shown on the left hand side. The current MV_(alv) value 80 is also shown. The lower portion of the exemplary display presents trend line 90 showing a target EtCO₂ over time (scale 74). The EtCO₂ magnitude scale 88 is shown in the lower left corner. The current EtCO₂ target value setting 84 is also shown below the current MV_(alv) value 80. In other embodiments, MV_(alv) may be replaced with minute volume or any other ventilation rate parameter. In such embodiments, the display may provide the ventilation rate parameter over time and the current value for the ventilation rate parameter in the same manner as shown and described in the exemplary embodiment of FIG. 5 .

In one embodiment, the control system may alert a clinician if the MV_(alv) value changes by more than a predetermined amount. The predetermined amount for the threshold MV_(alv) change may be adjustable, for example by an operator 17 setting the threshold value through input device 15 (FIG. 1 ). For example, the system may provide an alert or an alarm if the MV_(alv) value changes by more than 20% from a stable value.

FIG. 7 demonstrates another embodiment of a method for automatically ventilating a patient 35. The method 135 b is similar to the method 35 of FIG. 4 , with the exception of step 167. At step 167, the VCO₂ value and target EtCO₂ may be displayed to an operator, such as a clinician. For example, the display, such as display 12, may provide the VCO₂ value over time, and the display may be updated at step 167 to reflect the VCO₂ value. FIG. 8 depicts one exemplary display presenting a VCO₂ value and a target EtCO₂ value for a patient over time. The upper portion of the exemplary display presents a VCO₂ trend line 176 showing the VCO₂ value over a time scale 174. The VCO₂ magnitude scale 172 is shown on the left hand side. The current VCO₂ value 181 is also shown. The lower portion of the exemplary display presents trend line 90 showing a target EtCO₂ over time (scale 74). The target EtCO₂ magnitude scale 88 is shown in the lower left corner. The current EtCO₂ target value setting 84 is also shown below the current VCO₂ value 181.

In one embodiment, the control system may alert a clinician if the VCO₂ value changes by more than a predetermined amount. The predetermined amount for the threshold VCO₂ change may be adjustable, for example by an operator 17 setting the threshold value through input device 15 (FIG. 1 ). For example, the system may provide an alert or an alarm if the VCO₂ value changes by more than 20% from a stable value. In another example, an alert may be provided in response to how quickly the VCO₂ value changes. For example, in malignant hyperthermia the VCO₂ value may increase very rapidly. In air embolism, the VCO₂ value may decrease very rapidly. In one embodiment, the VCO₂ value may change, e.g., 50% in a 30 second period of time and an alert is provided.

The functional block diagrams, operational sequences, and flow diagrams provided in the Figures are representative of exemplary architectures, environments, and methodologies for performing novel aspects of the disclosure. While, for purposes of simplicity of explanation, the methodologies included herein may be in the form of a functional diagram, operational sequence, or flow diagram, and may be described as a series of acts, it is to be understood and appreciated that the methodologies are not limited by the order of acts, as some acts may, in accordance therewith, occur in a different order and/or concurrently with other acts from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology can alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all acts illustrated in a methodology may be required for a novel implementation.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

We claim:
 1. A method for automatically controlling ventilation of a patient, the method comprising: receiving a target expiratory CO₂ concentration set by an operator as a control parameter for a ventilator to automatically maintain a constant expiratory CO₂ concentration of the patient; measuring an actual expiratory CO₂ concentration; comparing the actual expiratory CO₂ concentration to the target expiratory CO₂ concentration; calculating an adjusted ventilation rate based on the comparison of the actual expiratory CO₂ concentration to the target expiratory CO₂ concentration in order to maintain the actual expiratory CO₂ concentration within a predetermined range of the target expiratory CO₂ concentration; automatically ventilating the patient with the ventilator based on the adjusted ventilation rate so as to maintain the actual expiratory CO₂ concentration for the patient within the predetermined range of the target expiratory CO₂ concentration so as to maintain the constant expiratory CO₂ concentration of the patient; and tracking the adjusted ventilation rate over time as an indicator of patient hemodynamic status.
 2. The method of claim 1, further comprising displaying the target expiratory CO₂ concentration and the adjusted ventilation rate over time.
 3. The method of claim 2, wherein the target expiratory CO₂ concentration and the adjusted ventilation rate over time are shown together with respect to a time axis.
 4. The method of claim 1, further comprising displaying the target expiratory CO₂ concentration and a carbon dioxide clearance value over time as an indicator of patient hemodynamic status.
 5. The method of claim 1, further comprising tracking the actual expiratory CO₂ concentration over time.
 6. The method of claim 5, further comprising displaying the adjusted ventilation rate over time and the actual expiratory CO₂ concentration over time.
 7. The method of claim 6, wherein the actual expiratory CO₂ concentration over time and the adjusted ventilation rate over time are shown together with respect to a time axis.
 8. The method of claim 5, wherein the actual expiratory CO₂ concentration over time and carbon dioxide clearance value are shown together with respect to a time axis.
 9. The method of claim 1, wherein the predetermined range is a predetermined percentage of the target expiratory CO₂ concentration.
 10. The method of claim 9, wherein the predetermined percentage is less than or equal to 0.5%.
 11. The method of claim 10, wherein the predetermined percentage is between 0.01% and 0.5%.
 12. The method of claim 1, wherein the expiratory CO₂ concentration of the patient is maintained constant based on the target expiratory CO₂ concentration set by the operator until a new target expiratory CO₂ is set by the operator.
 13. The method of claim 12, wherein, being held constant, the expiratory CO₂ concentration of the patient is not an indicator of patient hemodynamic status.
 14. The method of claim 1, wherein tracking the adjusted ventilation rate over time as an indicator of patient hemodynamic status comprises generating an alert in response to a determination that a change in an alveolar minute volume value or a carbon dioxide clearance value exceeds a predetermined change value.
 15. The method of claim 1, wherein tracking the adjusted ventilation rate over time as an indicator of patient hemodynamic status comprises generating an alert in response to a determination that a change in a carbon dioxide clearance value exceeds a predetermined rate of change value.
 16. A method for automatically controlling ventilation of a patient, the method comprising: receiving a target EtCO₂ for the patient set by an operator; receiving an alveolar minute volume value for the patient; measuring expiratory CO₂ in a gas expired from the patient; calculating an actual EtCO₂; comparing the actual EtCO₂ to the target EtCO₂; and detecting that the actual EtCO₂ is not within a predetermined range of the target EtCO₂, and then: adjusting the alveolar minute volume value based on the comparison; automatically ventilating the patient based on the adjusted alveolar minute volume value; and displaying a carbon dioxide clearance value over time to indicate the change in patient hemodynamic status.
 17. The method of claim 16, further comprising tracking the alveolar minute volume value over time; and displaying the target EtCO₂ and alveolar minute volume value over time together with respect to a time axis to indicate the change in patient hemodynamic status.
 18. The method of claim 16, further comprising tracking the alveolar minute volume value over time; and displaying the actual EtCO₂ and alveolar minute volume value over time together with respect to a time axis to indicate the change in patient hemodynamic status.
 19. The method of claim 16, wherein the target EtCO₂ for the patient is received from a user input device operated by an operator.
 20. The method of claim 16, wherein the predetermined range is a predetermined percentage of the target EtCO₂, wherein the predetermined percentage is between 0.01% and 0.5%.
 21. A system for automatically ventilating a patient, the system comprising: a ventilator; a gas analyzer; a controller; a display; wherein the controller is configured to: receive a target EtCO₂ for the patient set by an operator; receive an initial alveolar minute volume value for the patient; receive from a gas analyzer an actual EtCO₂ in a gas expired from the patient; compare the actual EtCO₂ to the target EtCO₂; detect that the actual EtCO₂ is not within a predetermined range of the target EtCO₂, and then: calculate an adjusted alveolar minute volume value; automatically control the ventilator to ventilate the patient based on the adjusted alveolar minute volume value so as to maintain the actual EtCO₂ concentration for the patient within the predetermined range of the target EtCO₂ concentration so as to automatically maintain a constant expiratory CO₂ concentration of the patient; and track an adjusted ventilation rate over time as an indicator of patient hemodynamic status.
 22. The system of claim 21, wherein the controller is further configured to continually calculate the adjusted alveolar minute volume value as required to maintain the actual EtCO₂ is within the predetermined range of the target EtCO₂.
 23. The system of claim 22, wherein the controller is further configured to operate the display to display the adjusted alveolar minute volume value with respect to time.
 24. The system of claim 22, wherein the controller is further configured to operate the display to display the adjusted alveolar minute volume value and the target EtCO₂ with respect to time.
 25. The system of claim 22, wherein the controller operates the display to display a carbon dioxide clearance value with respect to time.
 26. The system of claim 21, wherein the controller is further configured to store the adjusted alveolar minute volume value to memory. 